Studies have focused on macrophage heterogeneity and macrophage function, and on factors regulating NK activity. Monoclonal antibodies are being used to identify macrophage subpopulations. An IgM hybridoma antibody in the presence of complement eliminates macrophage accessory function but does not affect cytotoxic macrophages. Studies on the mechanism of suppression by macrophages have demonstrated that protein synthesis in lymphocytes was inhibited by activated macrophages, but not by elicited macrophages or a macrophage cell line. Lymphokine production could be blocked by transient inhibition of protein synthesis during the first 2 hours after stimulation or by irreversible inhibition of RNA synthesis at the initiation of culture. Induction of cytotoxic macrophages by macrophage activating factor (MAF) required the presence of endotoxin. Suppressor macrophages could be generated in vitro with MAF alone. Genetic studies on NK activity suggested that SJL/J mice have a single gene that determines low NK activity. Chemically modified Corynebacterium parvum could augment NK activity without suppression. Interferon-independent augmentation of NK activity was produced by lectins or antibodies.